The present invention relates to a method for manufacturing a rare earth magnet, more particularly, to a method for manufacturing a rare earth sintered magnet having an improved magnetic properties using rare earth alloy powder having a reduced oxygen content.
An Rxe2x80x94Fexe2x80x94B rare earth magnet (R is a rare earth element including Y) is mainly constructed of a major phase of an R2Fe14B tetragonal compound, an R-rich phase including Nd and the like, and a B-rich phase. The magnetic properties of an Rxe2x80x94Fexe2x80x94B rare earth magnet are improved by increasing the proportion of an R2Fe14B tetragonal compound as the major phase in the magnet.
The R-rich phase is required for liquid-phase sintering. Since R reacts with oxygen in an atmosphere to generate an oxide R2O3, part of R is wasted without serving for sintering. Therefore, an extra amount of R is required to replace the amount lost to oxidation. The oxide R2O3 is generated more vigorously as the concentration of oxygen is greater. In view of this, it has been attempted to reduce the concentration of oxygen in an atmosphere in which powder is produced to suppress generation of the oxide R2O3, and thus improve the magnetic properties of a sintered magnet.
The amount of oxygen in Rxe2x80x94Fexe2x80x94B alloy powder used for an Rxe2x80x94Fexe2x80x94B magnet is preferably as small as possible as described above. However, no method has succeeded in improving the magnetic properties by reducing the amount of oxygen in Rxe2x80x94Fexe2x80x94B alloy powder as a mass production technique. The reason is as follows. If Rxe2x80x94Fexe2x80x94B alloy powder is produced under an environment of a controlled low oxygen concentration and the amount of oxygen in the alloy powder is reduced to 4000 wt. ppm (mass. ppm) or less, for example, the powder may vigorously react with oxygen when the powder is exposed to the atmosphere (the air), causing the possibility of ignition in several minutes at room temperature. Moreover, in the case of adopting hydrogen processing for pulverizing, the alloy starts cracking from the rare earth-rich portions thereof. As a result, rare earth element tends to be exposed on the surfaces of pulverized powder particles. This further facilitates occurrence of ignition.
Therefore, while it has been recognized that the amount of oxygen in Rxe2x80x94Fexe2x80x94B alloy powder should desirably be reduced for improvement of the magnetic properties, it is extremely difficult to handle Rxe2x80x94Fexe2x80x94B alloy powder containing a low concentration of oxygen in a production site such as a plant.
In particular, the risk of ignition is high during a pressing or compacting process where powder is compacted in a press. In this process, the temperature of a compact rises due to heat generated by friction among powder particles during compaction and heat generated by friction between powder particles and the inner side wall of a cavity of the press during ejection of the compact. For prevention of ignition, the surroundings of the press may be put in a non-oxygen atmosphere. This is however unpractical because supply of the alloy powder and removal of the green compact are difficult. Occurrence of ignition may also be avoided if the compacts are swiftly subjected to sintering upon removal from the press. This is however extremely inefficient and thus not suitable for mass production. Also, in mass production facilities, it is difficult to manage compacts under an environment of an extremely low oxygen concentration through the processes from compacting to sintering.
A liquid lubricant such as fatty ester is often added to fines (fine powder) before compacting to improve compressibility or formability of the powder. By this addition of a liquid lubricant, thin oily coatings are formed on the surfaces of powder particles. Such coatings however fail to sufficiently prevent oxidation of the powder having an oxygen concentration of 4000 wt. ppm or less.
For the above reasons, a slight amount of oxygen is intentionally introduced into an atmosphere in which an Rxe2x80x94Fexe2x80x94B alloy is milled, to thereby oxidize thin surfaces of finely milled powder particles and thus reduce the reactivity of the powder. For example, Japanese Patent Publication No. 6-6728 discloses a technique as follows. A rare earth alloy is finely milled under a supersonic inert gas flow containing a predetermined amount of oxygen, so that thin oxide coatings are formed on the surfaces of powder particles produced by the milling. According to this technique, since oxygen in the atmosphere is blocked by the oxide coatings on the powder particles, occurrence of heat generation/ignition due to oxidation is prevented. However, with the existence of the oxide coatings on the surfaces of the powder particles, the oxygen content of the powder increases. This results in increase in the oxygen content (that is, the amount of a rare earth oxide generated) of a sintered body obtained after sintering. This may de-grade the magnetic properties of the resultant sintered magnet.
Japanese Laid-Open Patent Publication No. 10-321451 discloses a technique where Rxe2x80x94Fexe2x80x94B alloy powder having a small oxygen amount is mixed with mineral oil or the like to obtain slurry and a compact is produced from the slurry (wet compacting). Since powder particles in the slurry are kept from contact with the atmosphere, heat generation and ignition are prevented while the small amount of oxygen contained in the Rxe2x80x94Fexe2x80x94B alloy powder is maintained.
The wet compacting method is also adopted broadly for manufacture of ferrite magnets. In the manufacture of ferrite magnets, water is used for producing slurry. In the manufacture of Rxe2x80x94Fexe2x80x94B alloy magnets, however, use of water is difficult because Rxe2x80x94Fexe2x80x94B alloy powder reacts with water. This is the reason why an oil agent such as mineral oil is used. For Rxe2x80x94Fexe2x80x94B alloy magnets, a mineral oil or the like having comparatively low volatility is often used so as to reduce the amount of the oil agent that volatilizes from the slurry.
The above conventional technique has the following problem. After the Rxe2x80x94Fexe2x80x94B alloy powder in the slurry state is filled in a cavity of a press, the oil must be squeezed out during the pressing of the powder. This lowers productivity.
It is known that the magnetic properties degrade as the amount of carbon in an Rxe2x80x94Fexe2x80x94B sintered magnet increases. Therefore, in order to obtain a rare earth magnet having excellent magnetic properties after sintering, deoiling at high temperature is required to volatilize the oil agent used for formation of the slurry. In the above conventional technique, after production of a compact, the oil agent remains over the entire compact and moreover the amount of the oil agent contained in the compact is large. Therefore, it takes longer time to complete deoiling, and thus productivity decreases.
An object of the present invention is providing a method for manufacturing a rare earth magnet safely and efficiently using rare earth alloy powder having a low oxygen concentration.
The method for manufacturing an Rxe2x80x94Fexe2x80x94B rare earth magnet of the present invention includes the steps of: compacting rare earth alloy powder having an oxygen content of 4000 wt. ppm or less by dry compacting under compression to produce a compact; impregnating the compact with an oil agent from the surface of the compact; and sintering the compact.
The mean particle size of the rare earth alloy powder is preferably 10 xcexcm or less.
The rare earth alloy powder is preferably placed in an inert gas atmosphere having an oxygen concentration of 5000 vol. ppm or less until the powder is filled in a cavity of a press for production of the compact.
A vapor pressure of the oil agent is preferably 8 Pa or more at a temperature of 20xc2x0 C. The oil agent may be a volatile oil.
The temperature of the compact may be reduced at least temporarily due to volatilization of the oil agent.
The oil agent preferably includes a hydrocarbon solvent such as isoparaffin. A saturated hydrocarbon solvent is more preferable.
A lubricant is preferably added to the rare earth alloy powder before the compacting step.
The oil agent may be substantially removed from the compact before the compact is sintered. In this case, after the removal of the oil agent, the compact is preferably kept from contact with the atmosphere until the compact is sintered.
The removal of the oil agent is preferably performed under a reduced pressure at a temperature of 100 to 600xc2x0 C. for 0.1 to 8.0 hours.
In a preferred embodiment, after the compacting step, a surface temperature of the compact is measured, and the impregnation step is carried out only if the surface temperature is below a predetermined level. The surface temperature of the compact is preferably measured with an infrared thermometer. The compact of which the surface temperature is equal to or above the predetermined level is preferably stored in a sealable collecting box.
In a preferred embodiment, the impregnating step is carried out using an impregnation bath including openings through which the compact is put in and taken out and shutters for closing the openings.
A plurality of impregnation baths filled with the oil agent may be used, so that a predetermined number of compacts are individually put in the impregnation baths, to carry out the impregnation step.
At least one impregnation bath is preferably provided with a chiller for cooling the oil agent.
At least one impregnation bath is preferably provided with a thermometer for measuring the temperature of the oil agent.
In a preferred embodiment, after the impregnating step, the compact is placed on a sintering plate in an inert atmosphere before sintering. Alternatively, after the impregnating step, the compact may be stored in a sintering case in an inert atmosphere before sintering.
The xe2x80x9coil agentxe2x80x9d as used herein means hydrophobic liquid including a hydrocarbon solvent, a lubricant, and the like.